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| United States Patent |
6,141,395
|
|
Nishimura
, et al.
|
October 31, 2000
|
Sealed neutron tube
Abstract
A sealed neutron tube is provided, in which an insulating structure is
designed to be solid to enhance the shock-proof performance thereof, an
ion beam drawn out from an ion source is pulsated more rapidly, and the
lifetime of a target is increased without substantially increasing the
filling amount of the Tritium. The sealed neutron tube (1) includes a
metal housing (20), an ion source (5) disposed and sealed within the metal
housing for ionizing a Deuterium gas, an accelerating electrode (4)
charged with a high voltage, disposed and sealed within the metal housing
and facing the ion source, and a target (3) disposed within the
accelerating electrode and absorbing Tritium and the like therein. An
outer wall (20) is constructed by a metal housing, and a ceramic
insulating member (11) is disposed within the metal housing. Since the
accelerating electrode is held by this insulating member, the outer wall
of the sealed neutron tube has enhanced shock-proof performance. Further,
a permanent magnet (10) is disposed within the accelerating electrode, so
as to form a magnetic potential (a lateral magnetic potential) between the
target and the inlet of the accelerating electrode. Consequently, the
track of secondary electrons emitted from the target is bent, and
therefore the secondary electrons are prevented from leaking outside of
the accelerating electrode.
| Inventors:
|
Nishimura; Kazuya (Tokyo, JP);
Kato; Michio (Tokyo, JP);
Rintsu; Yuko (Tokyo, JP);
Nagakura; Masaaki (Saitama, JP);
Miyake; Yoshinobu (Saitama, JP)
|
| Assignee:
|
Japan National Oil Corporation (Tokyo, JP)
|
| Appl. No.:
|
260058 |
| Filed:
|
March 2, 1999 |
Foreign Application Priority Data
| Nov 25, 1998[JP] | 10-334217 |
| Current U.S. Class: |
376/114; 250/301; 376/108; 376/116 |
| Intern'l Class: |
G21B 001/00; G01T 001/167 |
| Field of Search: |
376/108,114,116
250/301,302,303
|
References Cited
U.S. Patent Documents
| 2973444 | Feb., 1961 | Dewan | 376/114.
|
| 3546512 | Dec., 1970 | Frentrop | 313/61.
|
| 3581093 | May., 1971 | Carr | 250/84.
|
| 3775216 | Nov., 1973 | Frentrop | 156/293.
|
| 3836785 | Sep., 1974 | Reifenschweiler | 250/501.
|
| 4310765 | Jan., 1982 | Givens | 250/493.
|
| 4675145 | Jun., 1987 | Kuswa et al. | 376/108.
|
| 4935194 | Jun., 1990 | Verschoore | 376/108.
|
| 4996017 | Feb., 1991 | Ethridge | 376/116.
|
| 5215703 | Jun., 1993 | Bernardet | 376/114.
|
| Foreign Patent Documents |
| 1425205 | Feb., 1965 | FR | 376/114.
|
| 1-312500 | Dec., 1989 | JP.
| |
| 1190991 | May., 1970 | GB.
| |
Other References
Reifenschweiler, "Neutrons from Small Tubes", Nucleonics, vol. 18, No. 12,
Dec. 1960, pp. 69-76.
Reifenschweiler, "Sealed-off Neutron Tube: The Underlying Research Work".
Philips Research Reports, vol. 16, No. 5, Oct. 1961, pp. 401-419.
|
Primary Examiner: Jordan; Charles T.
Assistant Examiner: Keith; Jack
Attorney, Agent or Firm: Wenderoth, Lind & Ponack, L.L.P.
Claims
What is claimed is:
1. A sealed neutron tube comprising:
a metal housing;
an ion source, disposed and sealed within said metal housing, for ionizing
a Deuterium gas;
an accelerating electrode charged with a high voltage, said accelerating
electrode being disposed and sealed within said metal housing and facing
said ion source;
a target disposed within said accelerating electrode and absorbing
Deuterium or Tritium therein; and
a insulating member for insulating said target and said accelerating
electrode from said metal housing,
wherein Deuterium ions produced by said ion source are accelerated by an
electric potential formed in a space between said ion source and said
accelerating electrode to collide with said target;
wherein nuclear fusion reaction is caused between said Deuterium ions and
said Tritium or said Deuterium absorbed in said target, to thereby
generate neutrons; and
wherein a magnet is disposed within said accelerating electrode so that
secondary electrons generated due to the collision of said Deuterium ions
with said target are prevented from leaking outside of said accelerating
electrode.
2. The sealed neutron tube according to claim 1, wherein said insulating
member is disposed between a high voltage power supply connecting rod
adjacent to said target, and said metal housing.
3. The sealed neutron tube according to claim 1, wherein said magnet is
disposed within said accelerating electrode in a cylindrical manner so as
to envelop said target.
4. The sealed neutron tube according to claim 1, wherein the ion source
comprises:
an ion source magnet;
a pair of ion source electrodes, one of said electrodes having an ion
outlet hole facing said accelerating electrode;
a plasma generation section, wherein a plasma is generated by synchronized
action of the magnetic potential formed by said ion source magnet and an
electric potential formed by said pair of ion source electrodes so that
ions of said pair of said plasma are extracted from said ion outlet hole;
and
a slit structure, wherein a plurality of metal plates alternately connected
to an electrical pulsed power supply and a ground are fixed in parallel,
said slit structure disposed in the vicinity of said ion outlet hole and
facing said accelerating electrode so that ions extracted from said ion
outlet hole can be interrupted in synchronism with said pulsed power
supply.
5. The sealed neutron tube according to claim 4, wherein one of said ion
source electrodes is at least one cathode and the other is an anode, and
said ion source is a cold cathode type ion source.
6. The sealed neutron tube according to claim 5, wherein said ion source
magnet is cylindrical, one of said cathodes is disposed at each end of
said cylindrical ion source magnet, said ion outlet hole is disposed at
the cathode that faces said accelerating electrode, and said anode is
disposed in a cylindrical manner within said cylindrical ion source magnet
and connected to a DC power supply.
7. The sealed neutron tube according to claim 1, wherein said target
includes a coin-like base, and a film of metal absorbing hydrogen formed
as a film of a metal absorbing hydrogen on said base, and a thickness of
said film of metal absorbing hydrogen is varied in proportion to a beam
density of said Deuterium ions irradiated onto said target.
8. The sealed neutron tube according to claim 7, wherein the thickness of
said film of metal absorbing hydrogen is increased from a peripheral
portion of said metal base toward a central portion thereof.
9. The sealed neutron tube according to claim 8, wherein said film of metal
absorbing hydrogen has a multi-layered structure such that layered
portions of said film of metal absorbing hydrogen have consecutively
smaller diameters from a central portion to the peripheral portion of said
metal base.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a structure for a sealed neutron tube
generating fast neutrons used in measurements, such as oil well logging
and the like, and in particular to a high voltage insulating structure, an
ion source structure and a target structure of the sealed neutron tube.
2. Description of the Related Art
A high voltage insulating structure, an ion source structure and a target
structure of a sealed neutron tube generating fast neutrons which is used
in measurements, such as oil well logging and the like, will be described
with reference to FIGS. 4 to 8.
As shown in FIG. 4, a sealed neutron tube 1 includes a cylindrical housing
wall 2, a target 3 absorbed Deuterium or Tritium therein inserted into the
housing wall 2, an accelerating electrode 4 charged with a high voltage,
and an ion source 5 for ionizing the Deuterium gas. A reservoir 6 in which
Deuterium is filled is attached to the ion source 5. The target 3 is made
contact through a high voltage connecting rod 7 to a high voltage power
supply (not shown).
The operating principle with the sealed neutron tube 1 thus constructed
will now be described. The Deuterium ions discharged in the ion source 5
are accelerated by the electric potential formed between the ion source 5
and the accelerating electrode 4 to collide with the target 3. This
collision causes nuclear fusion reaction between the Tritium or the
Deuterium absorbed in the target 3 and the Deuterium ions accelerated with
about 100 kV, to thereby generate neutrons.
Here, in order to accelerate the Deuterium ions extracted from the ion
source 5 to the accelerating electrode 4 within the sealed neutron tube 1,
an accelerating voltage of around 100 kV is applied between the ion source
5 and the accelerating electrode 4 (namely, the target 3). For this
reason, in a case where the ion source 5 side has a grounded electric
potential, the accelerating electrode 4 must be supported on the housing
wall 2 in an electrically insulated manner.
Therefore, as shown in FIG. 4, the housing wall 2 of the sealed neutron
tube 1 is constructed by an insulating member 8 formed of a ceramic or
glass, and the accelerating electrode 4 (namely the target 3) and the ion
source 5 are supported by this housing wall 2 to provide an insulating
structure. In FIG. 5, an alternative insulating structure for the sealed
neutron tube 1 is shown, in which only a part of the housing wall 2 (i.e.,
an accelerating electrode 4 side of the housing wall 2) is constructed by
the insulating member 8 formed of ceramic or glass to insulate the
accelerating electrode 4 from the housing wall 2.
Moreover, secondary electrons are emitted from the target 3 upon the
collision of the Deuterium ion beam, extracted and accelerated from the
ion source 5, with the target 3. These secondary electrons are attracted
to a grounded electric potential portion, such as the ion source 5, due to
the electric potential existing between the ion source 5 and the
accelerating electrode 4. Since a current which flows due to this
secondary electron emission results in a loss of energy which does not
contribute to the generation of neutrons, a structure is required to
suppress the secondary electrons emitted from the accelerating electrode
4.
Hence, as shown in FIG. 4, in order to suppress the secondary electrons
emitted from the accelerating electrode 4, a Faraday cap structure for the
accelerating electrode 4 is accommodated in the sealed neutron tube 1.
That is, the target 3 is enveloped by the accelerating electrode 4, and
the electric potential of the accelerating electrodes 4 is less than that
of the target 3 by about 500 to 2000 V. With the Faraday cap structure
constructed in this manner, the ion beam is collided to the target 3 while
being accelerated by the electric potential formed between the ion source
5 and the accelerating electrode 4, whereas the secondary electrons are
returned to the target 3 side by the electric potential formed between the
accelerating electrode 4 and the target 3 and are suppressed to be
released toward the ion source 5 side.
Next, an example of a cold cathode type ion source will be described with
reference to FIGS. 6 and 7. FIG. 6 illustrates an example of the ion
source 5 using a cylindrical magnet 51. The ion source 5 includes cathodes
52 attached to the ends of the cylindrical magnet 51, and a cylindrical
anode 53 disposed among the cathodes 52. A plasma generating section 55 is
defined in a space enveloped by the magnet 51, the cathodes 52 and the
anode 53. The cylindrical anode 53 is connected to a pulsed power supply
54. An ion outlet hole 56 is disposed at the cathode 52 where the ion
source 5 faces the target 3.
The generating principle of plasma with the ion source 5 thus constructed
will now be described. Initially, a magnetic potential is formed in the
axial direction by the magnet 51 within the ion source 5 and a voltage of
1 to 3 kV is applied to the anode 53. Next, the temperature of the
reservoir 6 (see FIG. 4) in which the Deuterium is absorbed is
subsequently raised to increase the gas pressure within the sealed neutron
tube 1 to about 10.sup.-3 to 10.sup.-2 mmHg. As a result, plasma is
generated in the plasma generating space 55 within the ion generating
source 5 by the synchronized action of the electric potential formed by
the anode 53 and the cathodes 52 and the magnetic potential formed by the
magnet 51. The positive ions in the plasma generated in the plasma
generating space 55 is extracted out of the ion outlet hole 56 by the
electric potential formed between the ion source 5 and the accelerating
electrode 4 (see FIG. 4). The positive ions thus extracted of the ion
outlet hole 56 form the ion beam and collide with the target 3 (see FIG.
4). In addition, in order to generate the neutrons in a pulse-shaped
manner, the plasma is intermittently generated within the ion source 5. To
this end, the pulse power supply 54 applies the pulsed voltage to the
anode 53 of the ion source 5.
FIG. 7 illustrates an example of the ion source 5 using a rod type magnet
57 as the magnet forming the magnetic potential. In this example as well,
the pulsed voltage is applied from the pulsed power supply 54 to the anode
53 of the ion source 5 to intermittently generate the plasma within the
ion source 5, thereby generating neutrons in a pulse-shaped manner.
Next, the target 3 which is generally used, will be described with
reference to FIG. 8. The target 3 includes a coin-like metal base 31 and a
film of metal absorbing hydrogen 32 coated on the metal base 31 by
processing such as sputtering of the metal absorbing hydrogen. The film of
metal absorbing hydrogen 32 is coated entirely on one side of the metal
base 31, or circularly coated on the one side of the metal base 31. The
thickness of the film of metal absorbing hydrogen 32 is about a uniform 1
to 10 .mu.m.
The sealed neutron tube to be used for oil well logging requires high
shock-proof performance because it operates in a bore hole. The sealed
neutron tube 1, however, has a problem in that the insulating housing wall
2 used, which is formed by an insulating member 8 of glass or the like,
has insufficient shock-proof performance. In the case of the housing wall
2 formed by an insulating member 8 of ceramic, the ceramic may be
dielectrically broken down and perforated by shocks, such that the sealed
neutron tube 1 breaks down. Further, the damage to the housing wall 2 will
result in leakage of the internally sealed tritium (a radioactive isotope)
out of the sealed neutron tube 1. That is, not only does the sealed
neutron tube 1 break down, but a serious problem is also caused in safety
handling. Therefore, there has been a demand to form the insulating
structure of the sealed neutron tube 1 as a firmer structure having
sufficient shock-proof properties, and in particular, as such a structure
which prevents the Tritium from being externally leaked even if the sealed
neutron tube 1 is damaged.
The pulsed neutron generating method adopted for the ion source 5 depends
on turning on and off the voltage applied to the anode 53 using the pulse
power supply 54. Therefore, it has been known that there is a slight time
lag between the application of the voltage to the anode 53 with the pulse
power supply 54 turned on and the stabilized generation of the plasma in
the plasma generating section 55. This time lag in the sealed neutron tube
1 is about 3 to 10 micro seconds. In contrast, when an inelastic
scattering .gamma.-ray is analyzed in oil well logging, as the generated
pulse width and pulse shape of the neutron beam generated in the
pulse-shaped manner becomes shorter and more accurate, respectively, the
accuracy of the logging used pulsed neutron becomes higher. Therefore, the
sealed neutron tube 1 used for the oil well logging requires an ion source
that is driven at a high speed in order to shorten the interval of the
pulse rate of the neutron burst.
Since the target 3 is irradiated by the Deuterium ions and the like
extracted from the ion source 5, the film of metal absorbing hydrogen 32
coated on the target 3 may be eroded due to the sputtering effect of the
ion beam. Therefore, protection against the shortening lifetime of the
target 3 due to this erosion is necessary.
It is conceivable, as a solution to protect against shortening lifetime, to
make the film of metal absorbing hydrogen of the target 3 more thick,
thereby making the lifetime of the film of metal absorbing hydrogen long.
However, since the film of metal absorbing hydrogen in an amount
substantially proportional to the increased amount of metal absorbing
hydrogen, it is necessary to increase the filling amount of the Deuterium
and Tritium in proportion to the thickness of the film of metal absorbing
hydrogen 32.
For example, in a case where Ti is used as the metal absorbing hydrogen,
the metal absorbing hydrogen Ti can absorb the hydrogen isotopic element
(i.e., Deuterium and Tritium in the case of the sealed neutron tube) in a
ratio substantially equal to the atomic ratio of the Ti and the hydrogen
isotopic element (Ti:hydrogen isotopic element=1:1.8). Accordingly, if the
target is 10 .mu.M thick and 12 mm in diameter, the weight of the metal
absorbing hydrogen Ti is 2.7 mg, which corresponds to 5.6.times.10.sup.-5
mol, and therefore there is a possibility that the Tritium and Deuterium
will be absorbed at 10.1.times.10.sup.-5 mol, that is, 1.8 times the
weight. Assuming that the Tritium shares 1/2 of the amount absorbed into
the target 3, this corresponds to 1.5 Ci (5.6.times.10.sup.10 Bq).
In contrast, from the standpoint of the waste disposal problem encountered
after the sealed neutron tube 1 is used, it is desirable that the amount
of Tritium, a radioactive base, used is made as small as possible.
Therefore, it is desirable to lengthen the lifetime of the target without
substantially increasing the amount of tritium to be absorbed.
SUMMARY OF THE INVENTION
The present invention is made in order to solve the problems encountered in
the related art. Accordingly, an object of the present invention is to
provide a sealed neutron tube which has higher shock-proof performance,
operates at a higher pulsing rate, and has a longer lifetime.
A sealed neutron tube according to a primary aspect of the present
invention includes: a metal housing; an ion source, disposed and sealed
within the metal housing for ionizing a Deuterium gas; an accelerating
electrode charged with a high voltage, the accelerating electrode being
disposed and sealed within the metal housing so as to face the ion source;
a target disposed within the accelerating electrode and absorbing
Deuterium or Tritium therein; and an insulating member for insulating the
target and the accelerating electrode from the metal housing, wherein
Deuterium ions produced by the ion source are accelerated by an electric
potential formed in a space between the ion source and the accelerating
electrode to collide with the target; wherein nuclear fusion reaction is
caused between the Deuterium ions and the Tritium or the Deuterium
absorbed in the target, to thereby generate neutrons; and wherein a magnet
is disposed within the accelerating electrode so that secondary electrons
generated due to the collision of the Deuterium ions with the target are
prevented from leaking out of the accelerating electrode.
It is preferable that the insulating member is disposed between a high
voltage connecting rod contacted to the target, and the metal housing. It
is desirable that the magnet is disposed within the accelerating electrode
in such a manner as to cylindrically envelope the target.
The sealed neutron tube according to another aspect of the present
invention is characterized in that the ion source has a magnet and a pair
of electrodes one of which has an ion outlet hole facing the accelerating
electrode, plasma is generated within the ion source by the synchronized
action of a magnetic potential formed by the magnet and an electric
potential formed by the pair of electrodes so that ions of the plasma are
extracted from the ion outlet hole, a slit structure in which a plurality
of metal plates alternately connected electrically to a pulsed power
supply and a ground are fixed in parallel to one another is disposed in
the vicinity of the ion outlet hole facing the accelerating electrode so
that the ions extracted from the ion outlet hole are interrupted in
synchronism with the pulsed power supply.
It is preferable that one of the electrodes be formed of a cathode or
cathodes, the other electrode be an anode, and the ion source is a cold
cathode type ion source. It is also preferable that the magnet be
cylindrical, with cathodes disposed on both ends of the cylindrical
magnet, the ion outlet hole disposed at the cathode facing the
accelerating electrode, and the anode disposed in a cylindrical manner
within the cylindrical magnet and connected to a DC power supply.
The sealed neutron tube according to yet another aspect of the present
invention is characterized in that the target includes a coin-like metal
base, and a film of metal absorbing hydrogen formed on the metal base, and
a thickness of the film of metal absorbing hydrogen is varied in
proportion to the beam density of the Deuterium ions collided onto the
target.
It is preferable that the thickness of the film of metal absorbing hydrogen
is increased from a peripheral portion of the metal base toward a central
portion thereof. In particular, it is more preferable that the film of
metal absorbing hydrogen has such a multi-layered structure such that
layered portions of the film of metal absorbing hydrogen are made with
consecutively smaller diameters.
In the sealed neutron tube according to the present invention, since the
housing wall is constructed by the metal tube and the accelerating
electrode is held within this metal tube by the provision of the ceramic
insulating member, the outer wall (the side wall) of the sealed neutron
tube has increased shock-proof performance and the sufficiently thick
ceramic ensures the insulating performance. Consequently, the sealed
neutron tube of the invention can be used under severer vibration and
shock conditions than conventional neutron tubes. Moreover, with this type
of insulating and supporting structure, it is difficult to apply an
electric potential between the accelerating electrode and the target to
prevent emission of the secondary electrons. Therefore, in the present
invention, a permanent magnet is disposed within the accelerating
electrode to provide a magnetic potential (a lateral magnetic potential)
between the target and the inlet of the accelerating electrode in place of
providing the electric potential. Consequently, the track of the secondary
electrons emitted from the target is bent, and thus the secondary
electrons are prevented from leaking out of the accelerating electrode.
For high pulsing rate operation of the ion source, a plurality of
parallelly arrayed metal plates electrically alternately connected to a
pulsed power supply and a ground are arranged in the vicinity of the ion
outlet hole 56 so as to provide a slit structure, whereas the DC voltage
is applied to the anode of the ion source so that plasma is constantly
generated. The change in the electric potential formed by the slit
structure disposed in the vicinity of the ion outlet hole makes it
possible to vary the direction of the ion beam emitted from the ion source
toward the target, thereby pulsating the ion beam incident on the target.
That is, depending on the state of the electric potential in the slit
structure, the direction of the motion of the ions is made linear so that
the ion beam is incident on the target, or otherwise bent laterally at 90
degree so that the ion beam to the target is interrupted. According to
this method, although the time period required to charge the slit
structure may be a cause of time-delay of the pulse, the charging time can
nevertheless be shortened remarkably, and therefore a higher pulsing rate
operation can be realized compared to the method in which the plasma of
the ion source is turned on and off. Consequently, the sealed neutron tube
of the invention can generate neutrons as a neutron pulse having an
accurate pulse-shape with a pulse width of less than 1 micro second, which
is difficult to generate with the conventional neutron tube, as well as
continuously generate neutrons.
To lengthen the lifetime of the target, a target is used such that the
thickness of the film of metal absorbing hydrogen is varied in proportion
to the beam density in view of the fact that the erosion of the film of
metal absorbing hydrogen on the target is proportional to the ion beam
density onto the film of metal absorbing hydrogen. With this structure,
the erosion cycle of the film of metal absorbing hydrogen can be made
entirely uniform from the central portion of the film of metal absorbing
hydrogen to the peripheral portion thereof. Therefore, the performance
thereof is not lowered until the film of metal absorbing hydrogen is
eroded completely. That is, the most efficient use can be offered with
respect to the metal absorbing hydrogen of the same amount as the
conventional amount. Consequently, a stabilized target performance can be
obtained with a lower sealing amount of tritium until the target is eroded
completely.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic diagram showing a structure of a sealed neutron tube
according to an embodiment of the present invention.
FIG. 2 is a schematic diagram showing a structure of an ion source
according to the embodiment of the present invention.
FIG. 3 is schematic plan and side views showing a structure of a target
according to the embodiment of the present invention.
FIG. 4 is a schematic diagram showing an example of an insulating structure
in a conventional neutron tube.
FIG. 5 is a schematic diagram showing another example of the insulating
structure in the conventional neutron tube.
FIG. 6 is a schematic diagram showing an example of a conventional ion
source in which a cylindrical magnet is used.
FIG. 7 is a schematic diagram showing another example of the conventional
ion source in which a rod type magnet is used.
FIG. 8 is schematic plan and side views showing an example of a
conventional target.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring to the accompanying drawings, a description will be given of the
preferred embodiment of the present invention which the inventor believes
to be the best mode at present.
In the following description and accompanying drawings, the same or
functionally equivalent component parts are denoted by the same reference
numerals. Note that the terms "right", "left", "up", "down" and the like
are used in the following description simply for the sake of convenience,
and the present invention should not be interpreted restrictively by these
terms.
FIG. 1 is a schematic diagram showing a structure of a sealed neutron tube
1 according to the present invention. As illustrated, the sealed neutron
tube 1 is provided with a metal housing 20 as an outer wall. Within the
metal housing 20, an ion source 5 for ionizing the Deuterium gas, an
accelerating electrode 4 charged with high voltage, a target 3 in which
Deuterium or Tritium is absorbed, and an insulating member 11 for
insulating the target 3 and the accelerating electrode 4 from the metal
housing 20 are sealingly disposed.
Specifically, the coin-like target 3 is disposed within the accelerating
electrode 4. A cylindrical magnet 10 for preventing secondary electrons is
disposed within the accelerating electrode 4 to enveloped the target 3
(the function thereof will be described latter). The target 3 is connected
through a high voltage introducing metal rod, i.e, a high voltage
connecting rod 7, to a high voltage power supply (not shown). The ion
source 5 side of the accelerating electrode 4 is open.
The insulating member 11 is disposed to support the high voltage connecting
rod 7 within the metal housing 20 while surrounding the connecting rod 7.
The target 3 and the accelerating electrode 4 are insulated from the metal
housing 20 by the insulating member 11. The insulating member 11 is made
of a ceramic. The accelerating electrode 4 is supported by the insulating
member 11 and disposed substantially coaxial to the metal housing 20.
Since the outer wall is formed by the metal housing and the ceramic
insulating member 11 is provided within the metal housing 20 to hold the
accelerating electrode 4, the outer wall (side wall) of the sealed neutron
tube 1 has enhanced shock-proof properties, and the ceramic of sufficient
thickness ensures insulating properties.
The ion source 5 is fixed through a reservoir 6, in which the Deuterium is
filled, to the metal housing 20 so as to be substantially coaxial to the
metal housing 20. Therefore, the ion source 5 and the accelerating
electrode 4 are disposed so as to confront or face each other. In order to
form an electric potential for accelerating the Deuterium ions radiated
from the ion source 5 toward the accelerating electrode 4, application of
a voltage of about 100 kV is required between the ion source 5 and the
accelerating electrode 4 (namely the target 3). Accordingly, in this
embodiment, the ion source 5 is used as the grounded electric potential,
whereas the accelerating voltage is applied to the accelerating electrode
4, and it is further necessary that the accelerating electrode 4 is
insulated from the metal housing 20. That is, since the insulating member
11 insulates the accelerating electrode 4 from the metal housing 20 as
described above, the accelerating voltage forming the electric potential
for accelerating the Deuterium ions drawn out of the ion source 5 can be
applied between the ion source 5 and the accelerating electrode 4.
The operating principle of the sealed neutron tube 1 thus constructed is
the same as that described in connection with the conventional neutron
tube. The Deuterium ions produced in the ion source 5 are accelerated by
the electric potential formed between the ion source 5 and the
accelerating electrode 4 to collide with the target 3. This collision
causes nuclear fusion reaction between the Tritium or the Deuterium
absorbed in the target 3 and the Deuterium ions colliding with the target
3, to thereby generate neutrons.
Moreover, secondary electrons are emitted from the target 3 upon the
collision of the Deuterium ion beam extracted and accelerated from the ion
source 5 with the target 3. The insulating and supporting structure as
mentioned above has difficulty in adopting the Faraday cap structure
providing an electric potential between the accelerating electrode 4 and
the target 3 for preventing the emission of the secondary electrons.
Therefore, in this embodiment, a permanent magnet 10 is disposed within
the accelerating electrode 4 to provide a magnetic potential (a lateral
magnetic potential) between the target 3 and the inlet of the accelerating
electrode 4 in place of providing the electric potential. Consequently,
the track of the secondary electrons emitted from the target 3 is bent,
and thus the secondary electrons are prevented from leaking out of the
accelerating electrode 4.
Next, with reference to FIG. 2, a structure of an ion source according to
the present invention will be described. FIG. 2 is a schematic diagram
showing a structure of a cold cathode type ion source 5. As illustrated,
the ion source 5 includes cathodes 52 attached, respectively, at the both
ends of the cylindrical magnet 51, and a cylindrical anode 53 disposed
between the cathodes 52. A plasma generating section 55 is defined in a
space enveloped by the magnet 51, the cathodes 52 and the anode 53. The
cathodes 52 and the anode 53 are connected to a DC power supply 61. An ion
outlet hole 56 is provided at the cathode 52 where the ion source 5 faces
the target 3. A plurality of metal plates 63 electrically connected to a
pulsed power supply 60 and a plurality of electrically grounded metal
plates 64 are alternately fixed in parallel at the accelerating electrode
4 side in the vicinity of the ion outlet hole 56 so as to form a slit
structure 62.
The generating principle of plasma with the ion source 5 thus constructed
will now be described. The magnet 51 forms the magnetic potential in the
axial direction within the ion source 5, whereas the DC voltage is
constantly applied by the DC power supply 61 to the anode 53 and the
cathodes 52, and therefore plasma is constantly generated in the plasma
generating section 55 within the ion source 5. The positive ions of the
plasma generated in the plasma generating section 55 are drawn out of the
ion outlet hole 56 by the electric potential formed between the ion source
5 and the accelerating electrode 4 (see FIG. 1).
In the embodiment, the change in the electric potential formed by the slit
structure 62 disposed in the vicinity of the ion outlet hole 56 makes it
possible to vary the direction of the ion beam emitted from the ion source
5 toward the target 3, thereby pulsating the ion beam incident on the
target 3. That is, when the output voltage of the pulsed power supply 60
is 0 V, the electric potential between the metal plates 63 and 64 in the
slit structure 62 are equal to each other, so that the ion beam can freely
pass from the ion source 5 through gaps in the slit structure 62 toward
the target 3. In contrast, when the output of the pulsed power supply 60
is positive, the electric potential between the metal plates 63 and 64 is
greatly inclined so that the track of the ion beam emitted from the ion
source 5 is bent toward the grounded metal plates 64, and the ion beam can
not pass through the gaps in the slit structure 62 if the metal plates 63
and 64 have a sufficiently large width. That is, the extraction of the ion
beam toward the target 3 is interrupted. According to this method,
although the time period required for charging the slit structure 62 may
be a cause for time-delay of the pulse the charging time can nevertheless
be shortened remarkably, and therefore higher pulsing rate operation can
be realized compared to the method in which the plasma of the ion source
is turned on and off.
FIG. 3 is a schematic diagram showing a structure of a target 3 according
to the present invention. As illustrated, the target 3 includes a
coin-like metal base 31, and a film of metal absorbing hydrogen 35 on the
metal base 31 which is formed as a thin film of the metal absorbing
hydrogen by a process such as sputtering. The thickness of the film of
metal absorbing hydrogen 35 is varied in proportion to the beam density of
the Deuterium ion beam emitted from the ion source 5 to the target 3.
Specifically, the metal base 31 in this embodiment is a Copper base, uses
Titanium as the metal absorbing hydrogen occlusion metal, and has a
multi-layered structure in which a plurality of circular layers are
stacked on one another on the Copper base. Since the ion beam density is
larger at the central portion of the target 3 than at the peripheral
portion thereof, the thickness of the film of metal absorbing hydrogen 35
is arranged to increase from the peripheral portion to the central
portion. The film of metal absorbing hydrogen 35 of the multi-layered
structure is formed so that a film of metal absorbing hydrogen portion 35a
which is directly contacted the metal base 31 has a large diameter, and
film of metal absorbing hydrogen portions 35b and 35c have consecutively
smaller diameters as they become distanced from the metal base 31. The
film of metal absorbing hydrogen 35 is formed on one side of the metal
base 31, and the thickness of each film portion of the film of metal
absorbing hydrogen is about a uniform 1 to 10 .mu.m.
In the target 3 thus constructed, the erosion cycle of the film of metal
absorbing hydrogen 35 due to the irradiation of the Deuterium ion beam can
be made entirely equal from the central portion to the peripheral portion
thereof. Therefore, the performance is not lowered until the film of metal
absorbing hydrogen is worn completely. That is, the most efficient use can
be offered with respect to the metal absorbing hydrogen of the same amount
as the conventional amount, and the lifetime of the target 3 can be made
longer.
Although the sealed neutron tube according to the present invention was
described with reference to the drawings, the present invention should not
be limited to this embodiment. For example, in place of the structure in
which the accelerating electrode 4 is insulated from and held in the metal
housing 20 by the insulating member 11 indirectly through the connecting
rod 7, the accelerating electrode 4 may be insulated from and held in the
metal housing 20 by the insulating member 11 directly such that the
cylindrical insulating member is disposed within the metal housing 20 to
enveloped the accelerating electrode 4.
Although the cold cathode type ion source is used as the ion source 5 in
this embodiment, the anode 53 and the cathodes 52 may be arranged in
reverse.
Although the cylindrical magnet 51 is used as the magnet for generating the
magnetic potential in the ion source 5, a rod type magnet may be used as
in the conventional example shown in FIG. 7.
Although the preferred embodiment of the present invention which the
inventor believes to be the best mode at present, and the modifications
thereof have been described with reference to the accompanying drawings,
the present invention should not be limited to these embodiments and
modifications. Various additional applications and modifications to a
sealed neutron tube can be easily made and realized without departing from
the spirit and scope of the present invention by one having ordinary skill
in the art.
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